1. Field of the Invention
The present invention generally relates to a high-resolution phased-array imager architecture and, more specifically, to a phase array in which the signals in a subset of elements or in all elements are multiplied by orthogonal or near-orthogonal signals.
2. Description of the Related Art
Passive imagers use the microwave and millimeter wave blackbody signals emitted from objects to form an image of a scene. In such imagers, the “angular resolution” refers to the ability to distinguish between objects that are separated by small angular distances.
In the case of a focal plane array, the angular resolution is proportional to the aperture size of the lens that is placed in front of the focal plane array. For example, in the case of a focal plane array shown in FIG. 1, the angular resolution is determined by the diameter of the lens. In such arrays, such angular resolution requires increased lens diameter and, furthermore, the lens has to be moved further from the focal plane array.
There are applications where high angular resolution is needed for imaging. For example, DARPA (Defense Advanced Research Projects Agency) is considering a Sandblaster imager with a 1 milli-radian resolution. The Sandblaster program is directed to permitting helicopter pilots to maintain vision in “brownout” conditions that force the pilot to attempt to land blind or unable to land at all, such as might occur under a helicopter's rotors especially in sandy or dusty areas.
It is noted that resolution ˜λ/D, where λ is wavelength and D is aperture. However, as λ is reduced or frequency is increased, the circuits become more difficult to implement, and increasing D also means that the physical size of the system increases. Still, given that one is limited by device performance, a higher D is attractive.
If a big lens (size ˜D) is used to focus the image on a focal plane imager, as exemplarily shown in FIG. 1, then the volume of the system increases. A lens is also difficult to manage and adds weight.
To eliminate the lens, a pupil-plane array can be built, such as a phased-array imager that is capable of beam-steering, as exemplarily shown in FIG. 2. In such an array, different phase and amplitude weights are applied to each element to create beams, using either all array elements or a subset of the elements. The phased array images the scene by steering the beam across the scene and the angular resolution is determined by the width of the beam. The beamwidth depends upon the size of the array (e.g., the largest spacing between any two elements) and the number of elements in the array.
However, the amplitude and phase information at each antenna should be known to construct the image, such as shown in the architecture of FIG. 3, wherein is incorporated digital or baseband beamforming simplifiers for phase shift and variable gain. However, this architecture requires separate mixers in each element, as well as a local oscillator (LO) source for the mixer.
The amplitude and phase of the signal at each antenna in the pupil plane can be gathered through a number of ways. For example, Lovberg, et al., describes a pupil plane array of antenna, amplifiers, and a Rothman lens to do the Fourier transform. A second method is shown in the system 400 shown in FIG. 4, in which N phased array elements provide a pupil plane array whose signals are combined for input into the receiver core.
However, such architectures involve high frequency signal distribution, combining, etc. Coherence must be maintained between the LO signals going into each array chip and the IF signals coming out of each array chip. Maintaining such coherence across chips that are placed in a large aperture is difficult and costly. Moreover, the combining process of FIG. 4 causes information to be lost.
Thus, the present inventors have recognized that a need continues to exist for improved methods to achieve high angular resolution for imaging.